LLC converters are a form of resonant converters. Resonant converters provide smooth waveforms, high efficiency, and high power density. Resonant converters are switching converters that include a tank circuit actively participating in determining input-to-output power flow. There are many types of resonant converters and most, if not all, resonant converters are based on a resonant inverter such as a system that converts a DC voltage into a sinusoidal voltage and provides power to a load. The sinusoidal voltage is typically a low harmonic content AC voltage. Generation of the sinusoidal voltage is achieved by a switch network that produces a square-wave voltage that is applied to a resonant tank circuit tuned to the fundamental component of the square wave. In this way, the tank responds primarily to the fundamental component and negligibly to the higher order harmonics, so that its voltage and/or current, are essentially sinusoidal.
LLC converters have been widely used in more and more applications for their ability to achieve high efficiency with soft switching. Direct frequency control is usually used for LLC converters, wherein a voltage loop compensator directly controls the switching frequency of the LLC converter. A small-signal transfer function of direct frequency controlled LLC converters varies with input voltage and load current. The transfer function changes between a 1st order system and a 2nd order system in different operating conditions, making the voltage loop compensator design very challenging and achieving very limited loop bandwidth. Another disadvantage of direct frequency control is that the line transient is poor because there is no direct control of the relationship between input current and output voltage